Orthodontic brackets are attached directly to teeth and serve to transmit corrective forces from an orthodontic archwire to the tooth to which the bracket is attached. The requirements of an orthodontic bracket are quite severe. First, it must have sufficient mechanical strength to withstand the forces to which it will be subjected. Exemplary forces are those transmitted by an archwire, by ligation, and by mastication. Second, the bracket must be chemically inert so that it will not corrode and also remain biologically compatible. Third, the bracket must be small enough to fit on the tooth.
Although there have been proposals for making orthodontic brackets from many different materials, the overwhelming majority of orthodontic brackets are still made of metal, usually stainless steel. Metal brackets meet all of these essential requirements but have one undesirable attribute --they are visible. A person undergoing orthodontic treatment has a conspicuous amount of metal in full view on the front surfaces of the teeth. And, since the treatment extends over a number of years, this appearance must be endured for a considerable period of time.
The incentive to make brackets from less unsightly materials has existed for many years. Recently, orthodontic treatment has given to an increasing number of adults, for whom the appearance of metal brackets is more than a mere annoyance. To avoid this unsightly visage, there have been proposed to make orthodontic brackets of less conspicuous materials, such as transparent or translucent plastics (such as polycarbonate), or ceramic materials which more closely resemble natural teeth. Also, it has been proposed to make these ceramic or plastic brackets with reinforcing metal inserts or liners for an archwire group. Most recently, there have been proposed orthodontic brackets comprising a crystalline alumina such as crystalline alpha-alumina. The strength and transparency properties of crystalline alumina and certain other crystalline alpha-alumina materials permit the provision of orthodontic brackets that are much more aesthetic than metal brackets but which alleviate to a large degree the strength limitations of plastic and ceramic brackets.
Nonetheless, there is still a problem with the crystalline alumina or "sapphire" orthodontic brackets. This is contained in the bonding mechanism for bonding the brackets to the teeth. That is, while the nonmetallic materials are desirable for use in providing aesthetic appearances, they are less than desirable in bonding to the teeth. This is true due both to inadequacies of the bond, and to the tendency of these nonmetallic brackets to remain bonded to the tooth even when it is desired to remove them from the mouth. This brings about a less than desirable result, from an aesthetic viewpoint when chemical debonding agents must be used to remove such stubbornly adhesive brackets.